Electrorefining is a key step in the pyrometallurgical process that was developed to recover uranium, plutonium, and minor actinides from spent metal fuel from the Integral Fast Reactor (IFR). The IFR is an advanced reactor concept that was developed at Argonne National Laboratory. Its distinguishing features are that it is a sodium-cooled, pool-type reactor (i.e., all the major components, reactor core, pumps, and heat exchangers are in a large sodium-filled pot); it employs a metallic fuel (an alloy of U, Pu, and Zr clad with a stainless steel-type alloy); and it has an integral fuel cycle (discharged core and blanket materials are processed and refabricated in an on-site facility).
Experiments performed with an engineering-scale electrorefiner have been reported in the literature, see for instance U.S. Pat. No. 5,009,752 issued Apr. 23, 1993 to Tomczak et al., assigned to the assignee hereof, the disclosure of which is incorporated herein by reference. The electrorefiner consisted of a cadmium anode or anodic dissolution baskets, solid and liquid cathodes, and a molten salt electrolyte (LiCl--KCl) at 500.degree. C. A dual cathode approach was adopted where uranium was recovered on a solid cathode mandrel and uranium-plutonium was recovered in a liquid cadmium cathode. In the engineering-scale electrorefiner, uranium has been electrotransported from the cadmium anode to a solid cathode in 10 kg quantities. Also, anodic dissolution of 10 kg batches of chopped fuel (U-10 wt % Zr) has been demonstrated. Development of the liquid cadmium cathode for recovering 4 kg of heavy metal has also been demonstrated.
In the electrorefining of spent IFR metal fuel to recover uranium and plutonium in the Fuel Cycle Facility, the concentrations of alkali, alkaline earth, and rare earth fission products in the salt in the electrorefiner must be reduced to lower the amount of heat generated therein. A heavy metal drawdown step (an operation to reduce the heavy metal concentration in the salt) is required before removing the fission products from the salt.
Previously, the technique for removing uranium and plutonium had involved a chemical displacement using a mixture of lithium and cadmium. As described in the attached document, tests with this procedure resulted in the formation of intermetallic compounds (solids) as a barrier to continued reaction. Dissolution of the intermetallic compounds required raising the melt temperature to 550.degree. C. from the normal 500.degree. C., which would be unacceptable in the electrorefiner because of increased cadmium vapor pressure.
This invention involves a method of electrochemically displacing the heavy metals by suspending a solid mixture of lithium and cadmium as an anode in the molten salt with a separate cathode, and applying voltage to drive the lithium and uranium (formed from a chemical reaction between the lithium and uranium chloride in the salt) into the salt and reduce the heavy metal or metals at the cathode. Tests have shown that the intermetallic barrier formed with the chemical method does not form with the electrochemical, thereby permitting U values to be reduced to parts per million.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.